Five young friends from Fresno, Calif., are nursing second-degree burns caused by a dangerous mix of two summertime staples: sun and citrus fruit.

Jewels, Jazmyn, Bailey, Candy and Reyghan, aged 7 to 11, were smashing limes with rocks and adding the juice to soda – a warm weather activity that seemed innocent enough. But the next day all five girls had blistering burns over huge swaths of skin.

"It hurt so bad," said 11-year-old Jewels, who described the pain as the "probably the worst" in her life. "It felt like there was a hundred needles just going in one spot."

The burns were so painful that the girls' parents rushed them to the hospital.

"A parent's worst nightmare is watching your kid scream and cry and begging you to stop the pain. And there is absolutely nothing you can do for your child," said Reyghan's mom, Melinda McDaniel.

All five girls were diagnosed with phytophotodermatitis, a form of skin irritation brought on by a reaction between photosynthesizing chemicals found in citrus fruits and ultraviolet light from the sun.

"UV light changes the structure of the chemicals and causes a toxic reaction on the skin," said Dr. Dawn Davis, a dermatologist at the Mayo Clinic in Rochester, Minn.

The severity of the reaction depends on the amount of chemical on the skin and the duration and intensity of sunlight exposure, according to Davis. And while the pain and inflammation typically subside in a matter of days, phytophotodermatitis can cause skin pigment changes that linger for weeks or months.

"We never thought lime can burn our skin like acid," said Jewels, whose sister Jazmyn was also suffered second-degree burns.

The burns can be seen on the girls' arms and legs, sometimes in patterns of splashes and drips.

"We often see streaking patterns because juices from fruits tend to drip," said Davis, adding that bartenders are particularly prone to phytophotodermatitis. "If you spill something that contains lime, you can see spill or splash mark on the skin."

The girls also have burns on their faces from "daring each other to drink the lime juice," according to 9-year-old Bailey Kinser, who joked about the strange name of their surprising condition. "I'm all, phytophotodermatitis what?"

But ABC News' chief health and medical editor Dr. Richard Besser said the condition is more common than people realize.

"It can be anywhere from a mild rash where you see darkening of the skin to something quite severe like big blisters like these girls had," he said. "The only advice around this is if you've been handling limes, wash your hands before you go out in the sun."

And wear sunscreen, which can act as a chemical barrier to the sun's UV rays and a physical barrier to lime juice, according to Davis.